The government last night admitted for the first time that aircraft suspected of being used by the CIA to transport detainees to secret interrogation centres had landed at British military airfields.

After months of refusing to answer questions from MPs or the media, it disclosed that two aircraft known to have been chartered by the CIA landed 14 times at RAF Northholt, west London, and RAF Brize Norton in Oxfordshire between October 2003 and May 2004.

One aircraft, a Boeing 737, was registered N313P, the other, a Gulfstream, was initially registered N379P and later as N8068V.

The flights were disclosed by Adam Ingram, the armed forces minister, in a letter to Sir Menzies Campbell, newly elected leader of the Liberal Democrats. Last week, the Liberal Democrats threatened to report the minister to the parliamentary ombudsman if he continued to refuse to answer detailed questions about flights suspected of being used for "extraordinary rendition" - the practice of sending detainees to camps where they were at risk of being tortured.

Sir Menzies was among a number of MPs of all parties, including Andrew Tyrie - the Conservative MP for Chichester who has set up a special parliamentary committee to investigate the flights - who have tabled questions about the flights since the Guardian published details of them last September.

In their replies, ministers have said that they either have no record of CIA flights since 1998, when they received four requests from the Clinton administration, or that records it might once have had had since been destroyed.

In his letter last night, Mr Ingram did not say the aircraft were used by the CIA, but the government has never denied they were. Mr Ingram also did not describe the purpose of the flights.

He insisted his disclosure was not "at odds with the foreign secretary's statements on the subject". Jack Straw has said that the government is "unaware" of any CIA flights landing in Britain or using UK airspace since 1998 and transporting terrorist suspects.

However, Michael Moore, the Liberal Democrat's new foreign affairs spokesman, said last night: "Another week, another grudging admission from the government. There is now a well-established pattern of partial disclosures after repeated denials."

He added: "These are disturbing revelations. Were these rendition flights or not? Ministers must urgently obtain answers from the US government."

Liberty, the civil rights group, is supporting an amendment to the civil aviation bill now going through the Lords, tabled by Baroness D'Souza. It would oblige the government to require any plane using UK airspace to land if it had information that it was being used for purposes of unlawful rendition.